In the development of liquid crystal display devices, the problems including limited viewing angle and slow reaction speed of liquid crystal have been gradually overcome. Color shift with the variation of viewing angle, however, remains a problem especially in the alternating process between the bright state and the dark state or the gray level transformation. Thus, although a liquid crystal display device displays the same image, colors observed in different viewing angles are different from each other. For example, when an image is observed as the blue sky within a small viewing angle centered near normal incidence, the same image is observed as the dark blue or light blue sky as the viewing angle is increased. Color shift is more obvious with the change of the viewing angle in the dark state. As used herein the term “viewing angle” refers to an angle made between an eye direction and a perpendicular at a given point on the display surface.
Conventional optical compensation in the dark state of the liquid crystal display device mainly includes two aspects. The first aspect is focused on the light leakage in different viewing angles when absorption axes of an upper polarizer and a lower polarizer are not orthogonal. The second aspect is focused on the phase retardation resulting from the liquid crystals of the liquid crystal display device in different viewing angles. An optical compensation film can be disposed on both sides of the liquid crystal panel to increase the contrast ratio of the liquid crystal display panel in a large viewing angle and to improve performance. For example, one or more polarizers may be combined with an optical compensation film such as an A-plate compensation film, a C-plate compensation film, or a biaxial compensation film to reduce the light leakage in a large viewing angle. As the biaxial compensation films work best, its use is more prevalent. The cost of the biaxial compensation film however precludes its use on both sides of the liquid crystal display panel; thus, this type of film is usually disposed on only one side of a liquid crystal display panel. That is, only the biaxial compensation film is disposed between the lower polarizer and the liquid crystal display panel. Although the biaxial compensation film improves the contrast ratio at large viewing angles, color shift in the dark state is still a problem. Thus, there continues to be a need for a liquid crystal display device that has an improved performance quality at a reasonable cost.